Method of manufacturing a semiconductor device

ABSTRACT

A method of manufacturing a semiconductor device which has a crystalline silicon film comprises the steps of forming crystal nuclei in a surface region of an amorphous silicon film and then growing the crystals from the nuclei by a laser light. Typically the crystal nuclei are silicon crystals or metal silicides having an equivalent structure as silicon crystal.

This application is a Continuation of Ser. No. 08/377,938, filed Jan.25, 1995, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device which has acrystalline semiconductor material and a method for manufacturing thesame.

BACKGROUND OF THE INVENTION

Thin film transistors (TFTs) are known which utilize a thin filmsemiconductor formed on a substrate. The TFTs are utilized in integratedcircuits, especially in an electro-optical device such as an activematrix type liquid crystal device as switching elements for each pixelor as driver elements in a peripheral circuit for driving active matrixelements.

Amorphous silicon films are readily available for TFTs. However, theelectrical characteristics of amorphous silicon films are low. For thisreason, it is desired to use semiconductor films having a crystallinity,that is, polycrystalline, microcrystalline silicon, monocrystallinesemiconductor or the like.

As a method for forming silicon films having a crystallinity(crystalline silicon, hereinafter), it is known to deposit an amorphoussilicon film first and then crystallize it by applying heat or lightenergy such as laser light.

However, in the case of using heat energy, it is necessary to heat asubstrate to a temperature 600° C. or higher for more than 10 hours. Forexample, a Corning 7059 glass which is generally used as a substrate foran active matrix type liquid crystal device has a glass distortion pointof 593° C. Accordingly, the crystallization through such a hightemperature heat treatment is not desirable for a glass substrate. Onthe other hand, a short pulse laser such as an excimer laser has anadvantage that it does not cause a distortion in a glass substrate.However, the uniformity of device characteristics is not so good in thecase of using a laser. The inventors of the present invention consideredthat this is because of a temperature distribution in a laser beam.

The inventors of the present invention investigated a method forpromoting a heat crystallization and a method for reducing a dispersion(ununiformity) in a laser crystallization in order to solve the problemsconcerning a crystallization of an amorphous silicon as discussed above.

With respect to the heat crystallization, it has been confirmed by theinventors that an amorphous silicon film can be crystallized through aheat treatment at 550° C. for 4 hours by depositing a small amount ofnickel, palladium, lead or the like on the silicon film.

As a method for introducing a small amount of the foregoing elements(i.e. a catalyst element for promoting crystallization), it is possibleto use a plasma treatment, evaporation and ion implantation. The plasmatreatment is a method in which a plasma of nitrogen or hydrogen isproduced using an electrode including the catalyst element in a parallelplate type or positive columnar type plasma CVD apparatus, thereby,adding the catalyst element into an amorphous silicon film.

However, it is not desirable if the foregoing elements exist in asemiconductor too much because reliability or an electrical stability ofa semiconductor device using such a semiconductor is hindered.Accordingly, the inventors have found that catalyst elements need to beused for crystallizing an amorphous silicon but it is desirable that aconcentration of the catalyst elements in the crystallized silicon filmbe minimized. In order to achieve this object, it is desirable to use acatalyst element which is inactive in a crystalline silicon, and toaccurately control the amount of the catalyst to be added into thesilicon film in order to minimize the concentration of the catalystelement therein.

The crystallization process using a plasma treatment for adding nickelas a catalyst was studied in detail. The following findings wereobtained as a result:

(1) In case of incorporating nickel by plasma treatment into anamorphous silicon film, nickel penetrates into the amorphous siliconfilm to a considerable depth before subjecting the film to a heattreatment;

(2) An initial nucleation occurs from the surface of the film in whichnickel is added; and

(3) When a nickel layer is formed on the amorphous silicon film by vapordeposition, the crystallization of an amorphous silicon film occurs inthe same manner as in the case of effecting the plasma treatment.

In view of the foregoing, it is assumed that not all of the nickelintroduced by the plasma treatment functions to promote thecrystallization of silicon. That is, if a large amount of nickel isintroduced, there exists an excess amount of the nickel which does notfunction for promoting the crystallization. For this reason, it is thepoint or the face of the silicon which contacts nickel that functions topromote the crystallization of the silicon at lower temperatures.Further, it is concluded that nickel has to be minutely dispersed in thesilicon in the form of atoms. Namely, it is assumed that nickel needs tobe dispersed in the vicinity of a surface of an amorphous silicon filmin the form of atoms, and the concentration of the nickel should be assmall as possible but within a range which is sufficiently high topromote the lower temperature crystallization.

A trace amount of a catalyst element capable of promoting thecrystallization of silicon can be incorporated in the vicinity of asurface of an amorphous silicon film by, for example, vapor deposition.However, vapor deposition is disadvantageous concerning thecontrollability of the film, and is therefore not suitable for preciselycontrolling the amount of the catalyst element to be incorporated in theamorphous silicon film.

Next, with respect to a dispersion in a characteristics occurring in alaser crystallization, the inventors of the present invention foundthrough experiments that this is caused by mainly the two reason, i.e.(1) a nonuniformity in a crystallinity due to a temperature distributionon a laser irradiated surface, and (2) the creation of crystal nucleibeing contingent. Specifically, a laser beam generally has an intensitydistribution in accordance with a gaussian distribution. The temperatureof an amorphous silicon film is also in conformity with thisdistribution. As a result, during a crystallization of amorphous siliconthrough melting or partial melting, the crystallization must start at aregion which has a lower temperature or a higher temperature dispersionthan other regions because a crystallization occurs when a region from amelting condition to a solid phase. However, in practice, a crystalnuclei does not necessarily exist in such a region and therefore, thereis a possibility that a supercooling region is formed. If such asupercooling region contacts crystal nuclei, a crystallization occursexplosively. Also, it is assumed that a uniform crystallization isdifficult because the crystal nuclei tend to be formed at a surfaceroughness of an interface with a silicon oxide.

Accordingly, it is desired that a region at which temperature firstlybecomes below a melting point among other regions is in conformity witha region in which crystal nuclei exist.

SUMMARY OF THE INVENTION

It is an object of the present invention to obtain a crystallinesemiconductor film with a high uniformity. More specifically, in view ofthe foregoing circumstances, it is an object of the present invention tocontrol the formation of crystal nuclei in a silicon film.

In accordance with one aspect of the present invention, crystal nucleiare introduced into at a predetermined region of an amorphous siliconfilm following which a laser crystallization is carried out. When thecrystal nuclei have a higher transmission rate with respect to the laserlight than the amorphous silicon and have a higher thermal conductivity,the temperature of the film becomes lower than the melting point firstat the crystal nuclei and therefore the crystallization starts there anda uniform crystalline film can be obtained. As the crystal nuclei, it isdesirable to use a material which allows an epitaxial growth of silicon,for example, minute crystallites of silicon, or nickel silicide which isformed by adding nickel to an amorphous silicon film and then heatingit.

Furthermore, it is desirable to add crystal nuclei not uniformly withina silicon film but on an upper or lower surface of the film. Theaddition of the crystal nuclei onto the surface of the silicon film isappropriate because the crystallization proceeds sufficiently in athickness direction of the film. Also, this is considered helpful forenlarging a size of each crystal.

Moreover, it is desirable to irradiate a laser light from the side ofthe silicon film on which the crystal nuclei are added. By doing so, itis possible to remarkably reduce a surface roughness after the laserirradiation as compared with the case in which only a laser irradiationis used without the formation of the crystal nuclei. The inventors ofthe present invention consider this is because the absorption efficiencyof the laser light by the crystal nuclei (i.e. crystal silicon) issmaller than that by an amorphous silicon so that this portion does notmelts easily. Surface roughness is comparable with that in the case ofusing only a solid phase growth. Generally, the surface roughness isdetrimental for a semiconductor device such as a TFT, for example, itcauses a scattering of carriers.

In accordance with one embodiment of the present invention, a method formanufacturing a semiconductor device comprises the steps of:

forming an amorphous silicon film;

introducing crystal nuclei to said amorphous silicon film; and

growing crystals from said crystal nuclei, thereby obtaining acrystalline silicon film.

The crystal nuclei are formed by adding a catalyst including a catalystelement such as nickel onto a surface of the amorphous silicon film andthen applying energy by heating or light irradiation (IR lightirradiation). Further, the crystals grow from the introduced crystalnuclei by irradiating a laser light or a light equivalent to the laserlight from the side on which the crystal nuclei are formed. The growthof the crystals is epitaxial.

As a method for adding a catalyst element, it is appropriate to coat anamorphous silicon film with a solution which contains the catalystelement therein. In particular, the catalyst element should be added bycontacting the surface of the amorphous silicon film. This is importantfor accurately controlling the amount of the catalyst element to beincorporated into the film.

The catalyst element may be added either from an upper surface or alower surface of the amorphous silicon film. In the former case, thesolution should be applied onto an upper surface of an amorphous siliconfilm after the deposition thereof. In the latter case, the solutionshould be applied onto a base surface and then the amorphous siliconfilm should be formed thereon.

The crystalline silicon film in accordance with the present invention issuitable as an active region of a semiconductor device which has atleast one electrical junction such as PN, PI, NI or the like. Forexample, thin film transistors, diodes, photosensors may bemanufactured.

The present invention has the following advantages:

(a) It is possible to accurately control and reduce the concentration ofa catalyst element in the silicon film.

(b) If the solution contacts a surface of an amorphous silicon film, theamount of the catalyst element to be incorporated into the silicon filmis determined by the concentration of the catalyst element in thesolution.

(c) It is possible to introduce the catalyst element into the amorphoussilicon film at a minimum density since the catalyst elements which areadsorbed by the surface of the amorphous silicon film function topromote the crystallization.

(d) A crystalline silicon film having a good crystallinity can beobtained without a high temperature process.

The catalyst provided by the solution may be in the form of a compoundor in the form of atoms. Also, it may be dissolved in the solution,alternatively, it may be dispersed in the solution.

In the case of using a polar solvent such as water, alcohol, acid orammonium, it is possible to use the following compounds for addingnickel, namely, nickel bromide, nickel acetate, nickel oxalate, nickelcarbonate, nickel chloride, nickel iodide, nickel nitrate, nickelsulfate, nickel formate, nickel acetyl acetonate, 4-cyclohexyl nickelbutyric acid, nickel oxide and nickel hydroxide.

Also, benzene, toluene, xylene, carbon tetrachloride, chloroform orether can be used as a non-polar solvent. Examples of nickel compoundssuitable for a non-polar solvent are nickel acetyl acetonate and 2-ethylhexanoic acid nickel.

Further, it is possible to add an interfacial active agent to a solutioncontaining a catalytic element. By doing so, the solution can be adheredto and adsorbed by a surface at a higher efficiency. The interfacialactive agent may be coated on the surface to be coated in advance ofcoating the solution.

Also, when using an elemental nickel (metal), it is necessary to use anacid to dissolve it.

In the foregoing examples, the nickel can be completely solved by thesolvent. However, even if the nickel is not completely solved, it ispossible to use a material such as an emulsion in which elemental nickelor nickel compound is dispersed uniformly in a dispersion medium. It isalso possible to use a solution which is for forming a silicon oxidefilm. An example of such a solution is OCD (Ohka Diffusion Source)produced by Tokyo Ohka Kogyo Kabushiki Kaisha. A silicon oxide film maybe easily formed by coating the OCD on a surface and then baking atabout 200° C. It is also possible to add desired impurities to thesilicon oxide film.

When using a polar solvent such as water for dissolving nickel, it islikely that an amorphous silicon film repels it. In such a case, a thinoxide film is preferably formed on the amorphous silicon film so thatthe solution can be provided thereon uniformly. The thickness of theoxide film is preferably 100 Å or less. Also, it is possible to add aninterfacial active agent to the solution in order to increase a wettingproperty.

When using a non-polar solvent such as toluene for obtaining a solutionof 2-ethyl hexanoic acid nickel, the solution can be directly formed onthe surface of an amorphous silicon film. However, it is possible tointerpose a material between the amorphous silicon film and the solutionfor increasing the adhesivity therebetween, for example, OAP (containinghexamethyl disilazane as a main component, produced by Tokyo Oka Kogyo)which is used to increase adhesivity of a resist.

The concentration of the catalyst element in the solution depends on thekind of the solution, however, roughly speaking, the concentration ofthe catalyst element such as nickel by weight in the solution should be0.01-10 ppm, preferably, 0.01-1 ppm. The concentration is measured basedon the nickel concentration in the silicon film after the completion ofthe crystallization.

After forming crystal nuclei by carrying out a heat treating on anamorphous silicon film which is added with a catalyst element, thesilicon film can be uniformly crystallized into a crystalline siliconfilm by the use of a laser irradiation.

When a laser crystallization is carried out on an amorphous silicon filmwithout crystal nuclei, the power of the laser necessary for thecrystallization is much higher than the laser crystallization in whichcrystal nuclei are previously formed. Conventionally, it was known thata laser power necessary for crystallizing an amorphous silicon filmhaving microcrystallites is higher than the laser power necessary forcrystallizing an amorphous silicon film having no crystallinity (becausethe difference in absorption efficiency of a laser light by the siliconfilms). However, the present invention is entirely opposite to thissince a lower laser power is sufficient for crystallizing a silicon filmin which crystal nuclei are formed.

In the present invention, the region of the silicon film which become acrystal nucleus upon crystallization can be controlled by controllingthe amount of the catalyst element incorporated into the film. The filmcan be regarded as in a state which is a mixture of a crystallinestructure and an amorphous structure. Typically, a proportion of crystalcomponents with respect to the entire plane of the film is from 0.01 to20%. By the application of a laser light in this state, crystals cangrow from the crystal nuclei which exist in the regions havingcrystallinity, and accordingly, it is possible to obtain a highercrystallinity. In other words, small crystallites are grown into largecrystallites. For this reason, the crystal growth length, the size andnumber of crystallites, or the like can be controlled by controlling theamount of a catalyst element and the power of a laser light.

Instead of using a laser light, it is also possible to use an intenselight, especially, an infrared light for crystallization. Since infraredray is not so absorbed by a glass substrate, it is possible to heat onlythe silicon film. This irradiation is generally called as a rapidthermal annealing (RTA) or rapid thermal process (RTP).

In the present invention, nickel is disclosed as a most preferredcatalyst element. However, it is to be understood that other catalystelements may be used in a similar manner. Examples of such elements arePd, Pt, Cu, Ag, Au, In, Sn, Pb, P, As and Sb. It is also possible toselect one or more elements from the groups VIII, IIIb, IVb and Vbelements of the periodic table.

In place of using a solution such as water or alcohol, it is alsopossible to use other materials which contain a catalyst material, forexample, metal compound or oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D show a manufacturing process in accordance with the presentinvention;

FIGS. 2A-2C show a manufacturing process in accordance with the presentinvention;

FIGS. 3A-3E show a manufacturing process of a TFT in accordance withEmbodiment 3 of the present invention;

FIGS. 4A-4D show a manufacturing process of a TFT in accordance withEmbodiment 6 of the present invention;

FIG. 5 is a block diagram showing an example of an active matrix liquidcrystal device in accordance with the present invention; and

FIG. 6 is a photograph corresponding to the cross sectional view of FIG.2A.

PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

Embodiment 1

In this embodiment, a crystallization through a laser irradiation aftera heat crystallization on a semiconductor film having been provided withan aqueous solution containing a catalyst will be discussed withreference to FIGS. 1A-1D.

It is desirable that a concentration of hydrogen contained in theamorphous silicon film before a laser crystallization be as small aspossible, for example, 0.01 to 10 atm %. For this reason, it ispreferable to heat the amorphous silicon film at a temperature which islower than a crystallization temperature thereof in order to effusehydrogen from the silicon film. In the alternative, it is possible toform an amorphous silicon film in which the concentration of hydrogenfalls within the above range by LPCVD using Si2H₆ (100-500 sccm) and He(500 sccm) at a relatively higher temperature for example 430 to 500° C.

In FIG. 1A, the reference numeral 11 shows a Corning 7059 glasssubstrate of 100 mm×100 mm. Initially, an amorphous silicon film 12 isdeposited by a known plasma CVD or LPCVD to a thickness of 100-1500 Å onthe substrate 11. For example, a plasma CVD is used and the thickness ofthe film is 1000Å.

The thus formed amorphous silicon film is treated with a hydrofluoricacid solution in order to remove contaminants or natural oxide from thesurface thereof, following which an oxide film 13 is formed to 10-50 Å.If it is possible to ignore the contaminants, a natural oxide may beused in place of the oxide film 13. The oxide film 13 should be verythin, for example, about 20 Å. The oxide film 13 is formed by anirradiation of UV light in an oxidizing atmosphere such as oxygen forabout 5 minutes. Alternatively, the oxide film may be formed by athermal oxidation or by treating it with a hydrogen peroxide. The oxidefilm 13 is for improving the wetting property of the surface, namely, anickel acetate acid solution which will be used to add a catalystelement in a later step can be uniformly coated on the entire surface ofthe silicon film by the provision of the oxide film 13. If there is nooxide film on the amorphous silicon film, the acetate solution tends tobe repelled by the amorphous silicon film so that it is not possible toadd nickel uniformly and therefore, it is not possible to perform auniform crystallization.

However, if a non-polar solvent such as a toluene solution of 2-ethylhexanoic acid nickel is used, the oxide film 13 is unnecessary and thesolution can be directly formed on the film 13.

Next, an acetate solution in which nickel is added is prepared. Theconcentration of nickel in the solution is 5 ppm. 2 ml of this solutionis dropped onto a surface of the oxide film 13 formed on the siliconfilm 12. This condition is maintained for 1-5 minutes. Then, a spin dryis performed by using a spinner at 2000 rpm for 60 minutes. This coatingstep may be repeated plural times if desired. As a result, a layer 14which contains nickel therein is formed on the amorphous silicon film 12to a uniform thickness of several Å to several hundreds Å. The nickel inthis layer will diffuse into the amorphous silicon film during a heattreatment and functions as a catalyst for promoting the crystallization.Also, this layer does not need to be a complete film, i.e. it may be adiscontinuous film.

The amorphous silicon film is maintained for 1-5 minutes after applyingthe solution thereto. The concentration of nickel introduced into thesilicon film may depend upon this period. However, the main factor fordetermining the concentration is the concentration of nickel containedin the solution.

Next, the substrate is heat treated in a nitrogen atmosphere for 1 hourat 550° C. As a result of this step, crystallinity is partly produced inthe silicon film 12, namely crystal nuclei are formed as shown in FIG.2A. Reference numeral 21 shows crystal nuclei formed in the amorphoussilicon film 12.

The temperature of the foregoing heat treatment should be not lower than450° C. If the temperature is lower than 450° C., the duration of theheat treatment should be lengthened so that the productivity is lowered.Also, if the temperature is higher than 550° C., a heat resistance ofthe glass substrate becomes a problem.

It is to be understood that the nickel containing solution may beapplied onto the substrate prior to forming the amorphous silicon film13. In such a case, the crystal nuclei are introduced from the bottomsurface of the amorphous silicon film.

After the foregoing heat treatment, the silicon film 12 is completelycrystallized by the irradiation of several shots of a KrF excimer laser20 (wavelength 248 nm, pulse width 30 nsec) in a nitrogen atmosphere.The power density of the laser is 200-350 mJ/cm². In place of the laser,it is possible to use an infrared light. It is important in thisinvention that the laser light is emitted from the upper surface of theamorphous silicon film on which the crystal nuclei have been formed.FIGS. 2A to 2C show how the crystals grow from the crystal nuclei 21 bythe laser irradiation. By this crystal growth, a polycrystalline film 23is formed. FIG. 6 is a photograph which corresponds to FIG. 2A. Smallcrystals which are grown from the crystal nuclei can be observed fromthe photograph of FIG. 6. Although there is a silicon oxide film betweenthe substrate and the silicon film, this can not be seen in FIG. 6.

Embodiment 2

This embodiment is entirely the same except that the concentration ofnickel in the solution is changed to 1 ppm.

The silicon film after the heat treatment was observed by a microscope.As a result, it was found that a portion of amorphous silicon isincreased as compared with the previous embodiment. Also, the number ofcrystal nuclei was reduced.

Further, after the laser crystallization, the sample was Secco-etchedand observed by SEM. As a result, it was found that the size of eachcrystal was larger than that obtained in the previous embodiment.

Embodiment 3

This embodiment relates to a process for fabricating TFTs which areprovided to each of the pixels of an active matrix liquid crystaldisplay device or for a driver circuit, using a crystalline silicon filmfabricated by the process according to the present invention. TFTs canbe applied not only to liquid crystal display devices, but also to awide field generally denoted as thin film integrated circuits (ICs).

Referring to FIGS. 3A to 3E, the process for fabricating a TFT accordingto the present embodiment will be described below. A silicon oxide film(not shown in the figure) is deposited to a thickness of 2,000 Å as abase coating on a glass substrate. This silicon oxide film is to preventthe diffusion of impurities into the device from the glass substrate.

An amorphous silicon film is deposited thereafter to a thickness of 500Å in the same manner as in Embodiment 1. After removing a natural oxidefilm by a hydrofluoric acid treatment, a thin film of an oxide film isformed to a thickness of about 20 Å by means of UV irradiation in anoxygen atmosphere.

The resulting amorphous silicon film having the oxide film thereon iscoated with an aqueous acetate solution containing nickel at aconcentration of 10 ppm. The resulting structure is retained for aduration of 5 minutes, and is subjected thereafter to spin drying usinga spinner. The silicon oxide film is removed thereafter using a buferredhydrofluoric acid, and a silicon film is partially crystallized byheating the resulting structure at 550° C. for a duration of 1 hour.Thus, a silicon film in which an amorphous components and crystallinecomponents are mixed is obtained. The crystallized portions function ascrystal nuclei.

Then, the silicon film is irradiated with a KrF excimer laser light fromthe upper surface of the silicon film at 200-300 mJ. During the laserirradiation, the substrate is heated at 400° C. Thus, crystals grow fromthe crystal nuclei which are formed in the former step.

The silicon film thus crystallized is patterned to form an island-likeregion 104 as shown in FIG. 3A. The island-like region 104 functions asan active layer of the TFT. A silicon oxide film 105 is formedthereafter to a thickness of from 200 to 1,500 Å for example, 1,000 Å.The silicon oxide film functions as a gate insulating film.

The silicon oxide film 105 is deposited by means of RF plasma CVDprocess using TEOS (tetraethoxysilane). That is, TEOS is decomposed andthen deposited together with oxygen at a substrate temperature of 150 to600° C., preferably in the range of 300 to 450° C. TEOS and oxygen areintroduced at a pressure ratio of 1:1 to 1:3 under a total pressure of0.05 to 0.5 Torr, while applying an RF power of 100 to 250 W.Alternatively, the silicon oxide film can be fabricated by reducedpressure CVD or normal pressure CVD using TEOS as the starting gastogether with gaseous ozone, while maintaining the substrate temperaturein the range of from 350 to 600° C., preferably, in the range of from400 to 550° C. The film thus deposited is annealed in oxygen or ozone inthe temperature range from 400 to 600° C. for a duration of from 30 to60 minutes.

A KrF excimer laser (operating at a wavelength of 248 nm with a pulsewidth of 20 nsec) or an intense light equivalent thereto may beirradiated in this condition in order to help the crystallization of thesilicon island 104. In particular, an application of RTA (rapid thermalannealing) using infrared radiation is effective because the siliconfilm can be heated selectively without heating the glass substrate.Moreover, the RTA is especially useful in the fabrication of insulatedgate field effect semiconductor devices because it decreases theinterface level between the silicon layer and the silicon oxide film.

Subsequently, an aluminum film is deposited to a thickness of from 2,000Å to 1 μm by electron beam vapor deposition, and is patterned to form agate electrode 106. The aluminum film may contain scandium at 0.15 to0.2 weight % as a dopant. The substrate is then immersed into anethylene glycol solution of a tartaric acid at 1-3% with its pHcontrolled to about 7 to effect anodic oxidation using platinum as acathode and the aluminum gate electrode as an anode. The anodicoxidation is effected by first increasing the voltage to 220 V at aconstant rate, and then holding the voltage at 220 V for 1 hour tocomplete the oxidation. During the condition of applying a constantelectric current, the voltage is preferably increased at a rate of from2 to 5 V/minute. Thus, an anodic oxide 109 is formed to a thickness offrom 1,500 to 3,500 Å, for example, 2,000 Å. (FIG. 6).

Impurities (phosphorus) are implanted into the island-like silicon filmof the TFT in a self-alignment manner by ion doping (plasma doping)using the gate electrode portion as a mask. Phosphine (PH₃) is used as adoping gas. The dose was from 1×10¹⁵ to 4×10¹⁵ cm⁻².

Thereafter, the crystallinity of the portion whose crystallinity isdamaged by the introduction of impurities is cured by irradiating a KrFexcimer laser (wavelength 248 nm, a pulse width of 20 nsec) as shown inFIG. 3C. The laser power density is from 150 to 400 mJ/cm², preferably,in a range of from 200 to 250 mJ/cm². As a result, N-type impurity(phosphorous) regions 108 and 109 are formed. The sheet resistance ofthe regions is found to be in the range of 200 to 800 Ω/square.

This step of laser annealing can be replaced by an RTA process, i.e., arapid thermal annealing process using a flash lamp in which thetemperature of the silicon film is rapidly raised to a range of from1,000 to 1,200° C. (as measured on the silicon monitor).

Next, a silicon oxide film is deposited to a thickness of 3,000 Å as aninterlayer insulator 110 through a plasma CVD using TEOS together withoxygen, or through reduced pressure CVD or normal pressure CVD usingTEOS together with ozone. The substrate temperature is maintained in therange of 250 to 450° C., for instance, at 350° C. A smooth surface isobtained thereafter by mechanically polishing the resulting siliconoxide film. (FIG. 3D).

The interlayer dielectric 110 is etched to form contact holes on thesource/drain as shown in FIG. 3E, and interconnections 112 and 113 areformed using chromium or titanium nitride.

In the case of a prior art method in which nickel is introduced by aplasma treatment, it is difficult to selectively etch only the siliconoxide film without etching the silicon film. However, in the presentinvention, nickel is incorporated into the silicon film by using anaqueous solution containing nickel at such a low concentration of 10ppm. Accordingly, a silicon film having a high resistance againsthydrofluoric acid can be formed and contact holes can be formed withhigh reproducibility.

Finally, the structure is annealed in hydrogen at a temperature of 300to 400° C. for 0.1 to 2 hours in order to hydrogenize the silicon film.Thus, the fabrication of the TFT is finished, which has source drainregions 108 and 109 and a channel region 114, and an NI junction 115. Aplurality of TFTs that are formed on the same substrate simultaneouslyin the foregoing process are arranged in a matrix to form an activematrix liquid crystal display device.

In accordance with the present example, the concentration of the nickelcontained in the active layer is assumed to be 3×10¹⁸ atoms/cm³ orlower, for example, in the range of 1×10¹⁶ to 3×10¹⁸ atoms/cm³.

The N-channel TFTs fabricated in this embodiment have a mobility of 200cm² /Vs or higher. Also, the threshold voltage of the TFTs is small.Further, it was confirmed that the variation in the mobility is within±10%. It is assumed that this is because uniform crystals are formed byuniformly introducing crystal nuclei through heat treatment and thencrystallizing by a laser irradiation. If only a laser irradiation isemployed, an N channel TFT having a mobility as high as 150 cm² /Vs canbe easily obtained. However, in such a case, the uniformity is not soimproved as in the present invention.

Embodiment 4

In this embodiment, crystal nuclei are introduced into an amorphoussilicon film from the side of a substrate and then a laser light isirradiated also from the side of the substrate for crystallization.

The structure of the TFT to be fabricated in this embodiment may be thesame as that disclosed in the previous embodiment. Since the substrateshould transmit a laser light, a quartz substrate is used in the case ofusing a KrF excimer laser (wavelength 248 nm). However, when a XeFexcimer laser of which wavelength is 353 nm or when other longerwavelength lasers are used, it is possible to use a glass substrate(Corning 7059).

Initially, a hydrofluoric acid treatment is carried out on the surfaceof a quartz substrate, following which the surface is exposed to a UVlight in an oxidizing atmosphere for 20 minutes. Since the surface ofthe quartz substrate tends to be hydrophobic due to contaminants, it isdesirable to carry out these steps in order to make the surface morehydrophilic and thereby a solution containing a catalyst can be formedmore uniformly.

The surface of the substrate is then coated with an acetate acidsolution containing nickel at a concentration of 10 ppm and is retainedfor a duration of 5 minutes, following which spin drying is done using aspinner. Thereafter, an amorphous silicon film having a thickness of 500Å is formed by plasma CVD or low pressure thermal CVD.

Then, the silicon film is partly crystallized by a heat treatment at550° C. for 1 hour in order to partly crystallize it. Thereby, crystalnuclei are formed on the lower surface (the surface which contacts thesubstrate). The crystal nuclei will function as crystal nuclei duringthe subsequent laser crystallization step. By this step, a silicon filmin which amorphous components and crystal components are mixed isobtained.

Next, a KrF excimer laser is irradiated from the substrate side at200-300 mJ in order to obtain a crystalline silicon film. The substrateis heated at 400° C. by using an infrared lamp during the laserirradiation. By this step, crystal growth is performed with the crystalnuclei as nuclei.

The thus formed crystalline silicon film is processed in the same manneras in Embodiment 3 to manufacture a TFT. The electrical characteristicsof the TFT manufactured in this embodiment is almost the same as that ofthe TFT obtained in Embodiment 3.

COMPARATIVE EXAMPLE

Instead of the heat treatment for forming crystal nuclei as explained inEmbodiments 3 and 4, the crystal nuclei are formed by an irradiation ofan infrared light having a wavelength of 1.2 μm. As a light source, itis possible to use a halogen lamp. The intensity of the infrared lightis controlled so that a temperature of a single crystalline siliconwafer which is a monitor is maintained within a range of 900 to 1200° C.More concretely, a thermocouple is embedded within a silicon wafer. Theoutput of the thermocouple is monitored and fed back to the lightsource. The temperature is raised at a constant rate of 50-200°C./second. Also, the film is naturally cooled at 20-100° C./second.Since the silicon film is selectively heated with this infrared lightirradiation, the damage to a glass substrate can be suppressed.

Embodiment 6

Referring first to FIG. 4A, a base film 502 of silicon oxide is formedon a Corning 7059 substrate 501 by sputtering to 2000 Å thick. Thesubstrate is desirably annealed at a temperature higher than adistortion point of the substrate following which the glass is cooled toa temperature less than the distortion point at a rate of 0.1-1.0°C./minute. Thereby, it is possible to reduce a contraction of thesubstrate due to a substrate heating which occurs later (for example,thermal oxidation, thermal annealing), accordingly, a mask alignmentprocess will be facilitated. This step may be performed either before orafter the formation of the base film 502 or it may be done both beforeand after the formation thereof. In the case of using the Corning 7059substrate, the substrate may be heated at 620-660° C. for 1-4 hours,following which it is cooled at a cooling rate of 0.1-0.3° C./minute andtaken out from a furnace when the temperature decreases to 400-500° C.

Then, an intrinsic (I-type) amorphous silicon film is formed to 500-1500Å thick, for example, 1000 Å through plasma CVD. The amorphous siliconfilm is provided with nickel as a catalyst for promoting acrystallization on its surface by the method described in Embodiment 1.Then, a heat treatment is performed at 550° C. for 1 hour in a nitrogenatmosphere (atmospheric pressure) in order to introduce crystal nucleiinto the surface of the silicon film. Further, the film is crystallizedby irradiating a KrF excimer laser. After the crystallization, thesilicon film is patterned into an island form having a dimension of10-1000 microns square. Accordingly, a crystalline silicon film 503 inthe form of an island is formed as an active layer of a TFT as shown inFIG. 4A.

Referring to FIG. 4B, the surface of the silicon film is oxidized byexposing the surface to an oxidizing atmosphere to form an oxide film504. The oxidizing atmosphere contains an aqueous vapor at 70-90%. Thepressure and the temperature of the atmosphere is 1 atm and 500-750° C.,typically 600° C. The atmosphere is produced by a pyrogenic reactionfrom oxygen and hydrogen gases with a hydrogen/oxygen ratio being1.5-1.9. The silicon film is exposed to the thus formed atmosphere for3-5 hours. As a result, the oxide film 504 having a thickness of500-1500 Å, for example, 1000 Å is formed. Since the surface of thesilicon film is reduced (eaten) by 50 Å or more due to the oxidation, aninfluence of a contamination of the upper most surface of the siliconfilm does not extend to the silicon-silicon oxide interface. In otherwords, by the oxidation, it is possible to obtain a cleansilicon-silicon oxide interface. Also, since the thickness of thesilicon oxide film is two times as the thickness of the portion of thesilicon film to be oxidized, when the silicon film is originally 1000 Åthick and the silicon oxide film obtained is 1000 Å, the thickness ofthe silicon film remaining after the oxidation is 500 Å.

Generally, the thinner a silicon oxide film (gate insulating film) andan active layer are, the higher a mobility is and the smaller an offcurrent is. On the other hand, a preliminary crystallization of anamorphous silicon film is easier when its thickness is larger. Moreover,an amorphous component or grain boundaries contained in the crystallinesilicon film tend to be oxidized during the thermal oxidation, resultingin a decrease in recombination centers contained in the active layer.Accordingly, the production yield can be improved.

After the formation of the silicon oxide film 504 through thermaloxidation, the substrate is annealed in a 100% monoxide dinitrogenatmosphere at 1 atm and 600° C. for 2 hours.

Referring to FIG. 4C, a polysilicon containing 0.01 to 0.2% phosphorousis deposited through low pressure CVD to 3000-8000 Å thick, for example,6000 Å, and then patterned into a gate electrode 505. Further, using thegate electrode 505 as a mask, an N-type impurity is added into a portionof the active layer in a self-alignment manner by ion doping (alsocalled as plasma doping). Phosphine is used as a dopant gas. Theacceleration voltage is 60-90 kV, for example, 80 kV. Also, the doseamount is, for example, 5×10¹⁵ cm⁻². Thus, N-type impurity regions 506and 507 are formed.

Thereafter, an annealing is performed with a KrF excimer laser(wavelength 248 nm, pulse width 20 nsec). The energy density of thelaser irradiation is 200-400 mJ/cm², for example, 250 mJ/cm². Also, thenumber of the shots of the laser light for one site is 2 to 10 shots,for example 2 shots. Further, the substrate may be heated at 200-450° C.during the laser irradiation.

The laser annealing may be replaced by a lamp annealing with a nearinfrared ray. The near infrared ray is absorbed by crystalline siliconmore effectively than by amorphous silicon. Accordingly, the annealingwith the near infrared ray is comparable with a thermal annealing at1000° C. or higher. On the other hand, it is possible to prevent theglass substrate from being detrimentally heated inasmuch as the nearinfrared ray is not so absorbed by the glass substrate. That is,although a far infrared ray can be absorbed by a glass substrate,visible or near infrared ray of which wavelength ranges from 0.5-4 μmare not so absorbed.

Referring to FIG. 4D, an interlayer insulating film 508 of silicon oxideis formed to 6000 Å thick by a plasma CVD. A polyimide may be usedinstead of silicon oxide. Further, contact holes are formed through theinsulating film. Electrode/wirings 509 and 510 are formed through thecontact holes by using a multilayer of titanium nitride and aluminumfilms. Finally, an annealing in a hydrogen atmosphere is conducted at350° C. and 1 atm for 30 minutes. Thus, a TFT is completed.

The mobility of the thus formed TFT is 110-150 cm² /Vs. The S value is0.2-0.5 V/digit. Also, in the case of forming a P-channel type TFT bydoping boron into source and drain regions, the mobility is 90-120 cm²/Vs and the S value is 0.4-0.6 V/digit. The mobility in accordance withthe present example can be increased by 20% or more and the S value canbe reduced by 20% or more as compared with a case where a gateinsulating film is formed by a known PVD or CVD.

Also, the reliability of the TFT in accordance with the present exampleis comparable to that of a TFT which is produced through a thermaloxidation at a temperature as high as 1000° C.

Embodiment 7

This embodiment, FIG. 5, shows an example of an integratedelectro-optical device (typically, a liquid crystal device) which hasone glass substrate on which all of a display, CPU and a memory areintegrally mounted. FIG. 6 shows a block diagram of such anelectro-optical device having TFTs in accordance with the presentinvention.

In the diagram, the input port is to read a signal input from an outsideand to convert it into a display signal. The correction memory which ispeculiar to each panel is to correct the input signal or the like inaccordance with a specific characteristics of an active matrix panel.Especially, the correction memory uses a non-volatile memory in which aninformation of each pixel is stored in order to perform the correctionat each pixel. That is, if there is a defective pixel (point defect) inthe electro-optical device, pixels surrounding the defective pixel aresupplied with signals which are corrected in order to disappear or coverthe defect pixel. Also, when there is a pixel of which brightness islower than others, the signal to be applied to that pixel is correctedto be a larger signal, thereby, the brightness of that pixel becomes thesame as its surrounding pixels. Pixels are provided with a liquidcrystal 73 and capacitor 72 and TFTs 71.

The CPU and the memory are equivalent to those used in a normalcomputer. Especially, the memory uses a RAM in which an image memorycorresponding to each pixel is stored. Also, it has a function ofchanging an intensity of back light on the rear side of the substrate inresponse to the image information.

While the preferred embodiments of the present invention have beendescribed, it is to be understood that many modifications may be made bythose ordinary skilled in this art without departing the scope of theappended claims. For example, the catalyst is added onto an entiresurface of an amorphous silicon film in the embodiments. However, it ispossible to add the catalyst only a selected region of a semiconductorfilm so that a higher crystallinity TFT and a lower crystallinity TFTmay be formed on one substrate or a higher crystallinity region and alower crystallinity region may be formed within one TFT.

What is claimed is:
 1. A method of manufacturing a semiconductor devicewhich has a crystalline semiconductor layer, said method comprising thesteps of:forming a non-single crystalline semiconductor layer comprisingsilicon; forming discrete silicon crystals within said semiconductorlayer by applying a first energy thereto; and then growing said discretesilicon crystals by applying a second energy to said semiconductor film,wherein said first energy is a different energy form from said secondenergy.
 2. A method according to claim 1 wherein said non-singlecrystalline semiconductor layer is amorphous.
 3. A method according toclaim 1 wherein said semiconductor layer is formed on an insulatingsurface.
 4. A method according to claim 1 wherein a concentration ofsaid catalyst in said semiconductor layer after the crystallization is1×10¹⁹ atoms/cm³ or lower.
 5. A method according to claim 1 wherein saidsecond energy is applied by heating and light.
 6. A method according toclaim 5 wherein said light is a laser light.
 7. A method according toclaim 5 wherein said heating is applied by a lamp.
 8. A method ofmanufacturing a semiconductor device which has a crystallinesemiconductor layer, said method comprising the steps of:forming anon-single crystalline semiconductor layer comprising silicon on aninsulating surface; providing said semiconductor layer with a catalystwhich is capable of promoting a crystallization of said semiconductorlayer; forming discrete silicon crystals within said semiconductor layerwhich is provided with said catalyst, by applying energy thereto; andthen growing said silicon crystals by irradiating said semiconductorlayer with light.
 9. A method according to claim 8 wherein said light isemitted from a laser.
 10. A method according to claim 8 wherein saidlight is emitted from a lamp.
 11. A method according to claim 8 whereinsaid non-single crystalline semiconductor layer is amorphous.
 12. Amethod according to claim 8 wherein said catalyst comprises a metal or asilicide thereof, said metal selected from the group consisting of Ni,Pd, Pt, Cu, Ag, Au, In, Sn, Pb, As, Sb.
 13. A method according to claim8 wherein said catalyst is applied onto an upper surface of saidsemiconductor layer.
 14. A method according to claim 8 wherein saidcatalyst is applied on a lower surface of said semiconductor layer. 15.A method according to claim 8 wherein said catalyst is applied to saidsemiconductor layer at a concentration of 1×10¹⁶ -1×10¹⁹ atoms/cm³. 16.A method according to claim 8 wherein said catalyst comprises at leastone element selected from VIII group elements, IIIb group elements, IVbgroup elements and Vb group elements.
 17. A method according to claims 1or 8 wherein a ratio of an area occupied by said discrete siliconcrystals with respect to an entire area of said silicon film is in therange of 0.01 to 20%.
 18. A method according to claim 8 further whereinsaid semiconductor layer is heated during irradiating with light.
 19. Amethod of manufacturing a semiconductor device comprising the stepsof:forming an amorphous silicon film by plasma CVD on an insulatingsurface; disposing a catalyst material including a metal in contact withsaid amorphous silicon film for promoting a crystallization of saidsilicon film; heating said amorphous silicon film in order to partlycrystallize said amorphous silicon film; and irradiating said siliconfilm with light after heating said amorphous silicon film so thatamorphous components remaining in said silicon film are crystallized.20. A method according to claim 19 wherein said metal is selected fromthe group consisting of Ni, Pd and Pt.
 21. A method according to claim19 wherein said metal is selected from the group consisting Cu, Ag andAu.
 22. A method according to claim 19 wherein said metal is In.
 23. Amethod according to claim 19 wherein said metal is Sn or Pb.
 24. Amethod according to claim 19 wherein said metal is As or Sb.
 25. Amethod according to claim 19 wherein a concentration of said catalystsaid semiconductor layer after the crystallization is 1×10¹⁹ atoms/cm³or lower.
 26. A method according to claim 19 wherein said light isemitted from a laser.
 27. A method according to claim 19 wherein saidlight is emitted from a lamp.
 28. A method according to claim 19 whereinsaid first energy is applied by a single energy source.
 29. A methodaccording to claim 19 wherein said second energy is applied by a singleenergy source.
 30. A method of manufacturing a semiconductor devicecomprising the steps of:forming a non-single crystalline semiconductorlayer comprising silicon on a substrate; forming discrete siliconcrystals within a portion of said semiconductor layer by applying afirst energy thereto; and growing said discrete silicon crystals byapplying a second energy so that said portion of the semiconductor layeris crystallized, wherein said portion of the semiconductor layerincludes an active region of said semiconductor device, and wherein saidfirst energy is a different energy form from said second energy.
 31. Amethod according to claim 30 wherein said active region is a channelregion.
 32. A method of manufacturing a semiconductor device comprisingthe steps of:forming a non-single crystalline semiconductor layercomprising silicon on a substrate; forming discrete silicon crystals inan upper portion of said semiconductor layer by applying a first energythereto; and performing a crystallization of said semiconductor layer byapplying a second energy thereto, wherein the crystallization of saidsemiconductor layer proceeds only from said discrete silicon crystalsthrough said semiconductor layer, and wherein said first energy is adifferent energy form from said second energy.
 33. A method ofmanufacturing a semiconductor device comprising the steps of:forming anon-single crystalline semiconductor layer comprising silicon on asubstrate through LPCVD; forming discrete silicon crystals within saidsemiconductor layer by applying a first energy thereto; and performing acrystallization of said semiconductor layer by applying a second energythereto, wherein the crystallization of said semiconductor layerproceeds only from said discrete silicon crystals through saidsemiconductor layer, and wherein said first energy is a different energyform from second energy.
 34. A method of manufacturing a semiconductordevice comprising the steps of:forming a non-single crystallinesemiconductor layer comprising silicon directly on a substrate; formingdiscrete silicon crystals within said semiconductor layer by applying afirst energy thereto; and performing a crystallization of saidsemiconductor layer by applying a second energy thereto, wherein thecrystallization of said semiconductor layer proceeds only from saiddiscrete silicon crystals through said semiconductor layer, and whereinsaid first energy is a different energy form from second energy.
 35. Amethod of manufacturing a semiconductor device comprising the stepsof:forming a non-single crystalline semiconductor layer comprisingsilicon on a quartz substrate; forming discrete silicon crystals withinsaid semiconductor layer by applying a first energy thereto; andperforming a crystallization of said semiconductor layer by applying asecond energy thereto, wherein the crystallization of said semiconductorlayer proceeds only from said discrete silicon crystals through saidsemiconductor layer, and wherein said first energy is a different energyform from said second energy.
 36. A method of manufacturing asemiconductor device comprising the steps of:forming a non-singlecrystalline semiconductor layer directly on a quartz substrate throughLPCVD; applying a catalyst containing material in contact with saidsemiconductor layer, said catalyst being capable promotingcrystallization of said semiconductor layer; forming discrete siliconcrystals in said semiconductor layer by applying a first energy thereto;and growing said discrete silicon crystals to crystallize saidsemiconductor layer by applying a second energy, wherein said firstenergy is a different energy form from said second energy.
 37. A methodaccording to claims 1, 30, 32, 33, 34, 35, or 36 wherein said firstenergy is applied by heating.
 38. A method according to claims 1, 30,32, 33, 34, 35, or 36 wherein said second energy is applied by a laser.39. A method according to claims 1, 30, 32, 33, 34, 35, or 36 whereinsaid second energy is applied by a lamp.